The modification to the controversial "backscatter" X-ray machine seeks to quiet critics such as the American Civil Liberties Union who have helped block its use in airports by saying the vivid images seen by screeners invade passenger privacy. The altered images will show only cartoon-like black-and-white outlines of each passenger.

But the Transportation Security Administration (TSA) is facing questions from experts who say the agency has weakened the effectiveness of the low-intensity X-rays.

"You don't see fine detail," said Robert Gould, a radiology professor at the University of California, San Francisco. "It's conceivable to me that (the modified machines) would miss plastic explosives."

Plastic explosives are a major threat because they evade checkpoint metal detectors and a small amount could blow apart a jet.

"I would bet that I could take some plastic explosives, put them on my upper thigh, and the blurring would let me get the stuff on board," said Andrew Karam, a physicist and fellow at the Center for Advanced Defense Studies, a national security think tank.

Backscatter X-rays will debut at Phoenix Sky Harbor International Airport, and a few other airports will follow. The TSA has lab-tested the machines and will use covert agents to test them in airports before any broader rollout. "We aren't endorsing this particular technology, we're testing it," TSA spokeswoman Ellen Howe said.

Manufacturer American Science and Engineering of suburban Boston got $722,000 from the TSA to program its backscatter machine so body parts wouldn't be visible to security screeners. The machines will be used only on passengers who require extra screening beyond a metal detector and who opt for the X-ray over a pat-down.

American Science Vice President Joe Rice said the $100,000 machine finds ceramic weapons and small metal objects that metal detectors can miss. "I'm sure there are ways to defeat this technology," Rice said. "But this type of technology is a dramatic leap forward from what is available today."